Characterization of the surface energies of functionalized multi-walled carbon nanotubes and their interfacial adhesion energies with various polymers
Graphical abstract
Introduction
The extraordinary properties of carbon nanotubes (CNTs) have attracted great interest for their application as polymer reinforcement [1], [2], [3]. Polymer composites reinforced with CNTs can exhibit superior mechanical properties relative to conventional composites because of the exceptional high specific stiffness and strength and their fiber-like structure. The key aspects contributing to the performance of the polymer/CNT composites include the extent to which CNTs can be dispersed and wetted by a given polymer. Wetting of CNTs by the polymer and good dispersion of CNTs in the polymer matrix are necessary to couple the inherent strength of CNTs to the polymer matrix [4], [5], [6], [7]. The interfacial adhesion between CNTs and the polymer matrix plays a crucial role in the physical performance of the composite.
Since the interfacial adhesion between CNTs and the polymer matrix depends on the surface energies of CNTs and the polymer, characterization is essential for screening polymer/CNT combinations [5]. The surface energies of various polymers are well characterized; however characterization of the surface energies of CNTs is still challenging. The surface energies of CNTs determined previously by various methods ranged from 27.0 to 45.3 mJ/m2 [5], [6], [7], [8]. The surface energy of a particular solid is generally estimated from the contact angle data between a particular solid and various liquids [9]. Three different methods are generally used to determine contact angles: sessile-drop, capillary-rise and drop-on-fiber [4], [5], [6], [7], [8], [9]. Each method has its own merits and limitations. Accurate contact angles between a particular solid and various liquids can be measured easily with the sessile-drop method when the solid surface is evenly flat, without pores. When the solid surface is rough and contains pores, a true equilibrium contact angle may not be obtained because the surface roughness and pores can change the contact area between the solid and liquid [10], [11]. Direct measurement of the contact angle is not possible for powdered solids [10], [11], [12], [13], [14]. For such solids, the capillary-rise method, in which change in liquid height is measured as a function of time, is most frequently applied [8], [12], [13], [14]. In comparison to contact angles measured directly on a smooth surface of the same solid, the capillary-rise method leads to overestimated contact angles [14]. To determine the contact angle of the liquid on a fiber-type solid, cylindrically symmetric drop-on-fiber systems are used [15], [16], [17], [18]. In this method, the whole drop profile is used for determination of the contact angle [15], [16], [17], [18]. However, application of this method is limited when gravity effects exist and liquid has poor wettability on CNTs [7], [16], [17]. Liquid droplets which do not envelop the CNTs (clam-shell type droplets) are formed when liquid has a poor wettability on CNTs [7], [16].
The surface modification of CNTs was initiated to enhance their adhesion with the polymer [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. For the screening of polymer/functionalized CNT combinations exhibiting improved interfacial adhesion, surface energies of the functionalized CNTs should be characterized. In this study, multi-walled carbon nanotubes (MWCNTs) covalently bonded with various functional groups were prepared, and changes in their surface energies with functional groups were characterized. To determine the proper method to estimate the surface energy of MWCNTs, the surface energy of pristine MWCNTs was explored by three different methods: sessile-drop, capillary-rise and drop-on-fiber. The surface energies of the functionalized MWCNTs were also estimated with the selected method, i.e., drop-on-fiber, and then characteristics of polymer composites containing pristine MWCNTs and functionalized MWCNTs were explored.
Section snippets
Materials
The MWCNTs used in this study were supplied by Nano Carbon Technologies Co., Ltd. (Kawasaki, Japan). According to the supplier, the MWCNTs had diameters ranging from 40 to 60 nm and an average purity of 95%. Nitric acid (HNO3, high performance liquid chromatography (HPLC) grade) and sulfuric acid (H2SO4, HPLC grade) used for the acid treatment of MWCNTs, thionyl chloride (SOCl2) used as coupling agent to form acyl chloride on the MWCNT surface and ethyl diamine (EDA) were purchased from Aldrich
Characterization of functionalized MWCNTs
XPS analyses were performed to confirm the formation of MWCNT-COOH, MWCNT-COCl and MWCNT-EDA. Fig. 2 shows XPS wide scans of pristine MWCNTs, MWCNT-COOH, MWCNT-COCl and MWCNT-EDA (Fig. 2a), as well as the fitted C1s bands of pristine MWCNTs (Fig. 2b) and MWCNT-COOH (Fig. 2c). The Cl2p peak of MWCNT-COCl (Fig. 2d) and the N1s peak of MWCNT-EDA (Fig. 2e) are also provided. As shown in Fig. 2a, a Cl2p peak from the acyl halide groups was observed in the spectrum of MWCNT-COCl. An N1s peak
Conclusion
To explore the interfacial adhesion behaviors of MWCNTs with various polymers, the surface energies of pristine MWCNTs were estimated from contact angle values measured by three different methods: drop-on fiber, capillary-rise and sessile-drop. A reliable surface energy value of pristine MWCNTs was obtained with the drop-on-fiber method. Surface energies of MWCNT-COOH, MWCNT-COCl and MWCNT-EDA were also estimated with this method. When the surface energy of pristine MWCNTs was compared to those
Acknowledgments
This research was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Knowledge Economy, Republic of Korea.
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